Orifice, Pitot & Other Differential Pressure (DP) Flow

Orifice Plates

Method

Based on Bernoulli’s Principle, an increase in flow speed results in a decrease in pressure. This principle is utilised in differential pressure (DP) elements like orifice plates. An orifice plate is a precisely machined disc placed in the pipeline, featuring a central hole restricting flow.

When the fluid or gas reaches the orifice plate, it is forced through this narrow opening. This constriction accelerates the flow and reduces the pressure on the downstream side, creating a pressure difference. The flow rate can then be calculated from this pressure difference.

Advantages

Suitable for measuring both liquids and gases
Highly accurate
No moving parts
Cost-effective

Disadvantages

Not suitable for fluids with suspended solids or dirty gases
Precision is key; most orifice plates are custom designed for specific applications
Requires stable pressure and temperature for accurate measurements in gas applications, unless using a multi-variable transmitter

Considerations

The introduction of an orifice plate causes a permanent pressure to drop in the system. For gas flow applications, multi-variable transmitters can measure differential pressure, static pressure, and process temperature, compensating for any changes. These advanced transmitters can also account for the expansion or contraction of pipework, ensuring exceptional accuracy in gas flow measurements.

Note: The example images provided are for liquid flows. For gas flows, transmitters should be positioned at the top of the pipeline.

Averaging Pitot Tubes

Method

Averaging pitot tubes operate on the same principle as orifice plates by creating a differential pressure directly related to the flow rate. However, they use a different design. Instead of cutting the pipe completely as required for an orifice plate, pitot tubes are insertion devices. They are installed by drilling one or two tapping points into the pipe and inserting the tube.

Pitot tubes are specifically engineered to generate a significant differential pressure, making them a great alternative when a full plate installation is not desired. They come in various shapes and sizes, optimised for different applications to enhance sensitivity.

Advantages Compared to Orifice Plates:

Easier to install, insert, and remove
Significantly lower pressure drops

Disadvantages Compared to Orifice Plates:

More sensitive to dust, dirt, or other debris in the process
Not suitable for some high-velocity applications
Generate lower differential pressures than orifice plates

Considerations

While averaging pitot tubes cause a much lower pressure drop compared to orifice plates, they still result in some system pressure reduction. For installations expecting high flows or pressure shocks, it is recommended to secure the tube on both sides of the pipe using a socket on either side of the unit.

Averaging pitot tubes can also come with a manifold at the top, allowing direct mounting of the transmitter. Additionally, multivariable transmitters are compatible with pitot tubes, enabling precise measurement and compensation for varying conditions.